The present invention relates to a so-called "oblique incidence vacuum deposition" method in which a magnetic material is applied by vacuum deposition obliquely to a flexible belt-shaped support being continuously conveyed to thereby manufacture a magnetic recording material.
Recently, as the demand for high-density magnetic recording has increased, instead of the conventional magnetic recording medium manufacturing method in which a binder type magnetic solution is coated on a flexible support and dried, a variety of non-coating system magnetic recording medium manufacturing methods in which, without using a binder, a ferromagnetic metal film layer is formed on a support by vacuum deposition, sputtering, ion plating, or the like have become of increased interest.
Among the non-coating magnetic recording medium manufacturing methods, an oblique incidence vacuum deposition method in which beams of evaporated magnetic metal are applied obliquely to the surface of a support so as to be desposited thereon has been found to be quite practical because the process is simple and the apparatus needed for implementing the method is relatively compact. Moreover, the method provides a magnetic film layer having excellent magnetic characteristics.
A specific feature of conventional oblique incidence vacuum deposition is that, while the support is being conveyed straightly or along the outer wall of a cylindrical drum, i.e. along a curved line, above the evaporating source, a ferromagnetic metal film is vacuum-deposited in one step on the support surface to a predetermined thickness by the evaporated metal beam from the evaporating source with the application (incident) angle of the beam being strictly limited. However, as the evaporated beam is oblique with respect to the support surface, the thickness of the vacuum-deposited metal film is equal to the cosine of the angle of incidence multiplied by the thickness of a vacuum-deposited metal film which would be formed with a zero incidence angle (where the evaporated metal beam forms a right angles with the support surface). Therefore, it is unavoidable that as the incident angle is increased, the efficiency of vacuum deposition is decreased. In addition, if the geometrical arrangement of the support and the evaporating source is such that the incident angle is increased, then the distance between the support and the evaporating source is correspondingly increased, and accordingly the efficiency of vacuum deposition is further decreased. Since the magnetic characteristics of the vacuum-deposited magnetic film depend on the incident angle (see, for example, Japanese Published Patent Application No. 352,558 - 1964), it is essential that the incident angle be as small as possible and that it be maintained unchanged during coating operations.
A low efficiency of vacuum deposition makes it difficult to decrease the manufacturing cost when relatively expensive nonferrous metal such as cobalt or cobalt alloy is used. Accordingly, this has been a serious problem requiring solution before the method can be put to practical use.
In order to solve the problem, for instance, as proposed in Japanese Patent Laid-Open Application No. 9607/1979, a method can be used in which the evaporating source is shifted from the position immediately below the drum so that only a high-density portion of the evaporated metal beam is applied to the surface of the support on the outer wall of the drum. Using this method, the efficiency of vacuum deposition is 20%.
However, with this method, it is difficult to increase the range of suitable evaporated metal beam incident angles with respect to the support on the outer wall of the drum. Accordingly, further improvement of the efficiency of vacuum deposition for a given evaporating source is limited. Further, the method must be significantly improved if the efficiency of vacuum deposition is to be made satisfactory. The method is also disadvantageous in that the resultant magnetic recording medium does not have uniform magnetic characteristics if an incident angle larger than a particular angle is used.
Accordingly, an object of the invention is to provide a magnetic recording medium manufacturing method in which all of the above-described problems relating to the efficiency of vacuum deposition of the conventional oblique incidence vacuum deposition method have been eliminated.
It is a further object of the invention to provide a method for manufacturing a magnetic recording medium having a high anti-magnetic force and high rectangular ratio.